Vibrato circuit



- July 12, 1966 w. H. KRUG 3,260,785

VIBRA'I'O CIRCUIT Filed Oct. 17, 1963 2 Sheets-Sheet 2 J5 J0 y 1 UnitedStates Patent 3,260,785 VIBRATO CIRCUIT William H. Ifiug, 1221 JeromeAve., Janesville, Wis. Filed Oct. 17, 1963, Ser. No. 316,886 9 Claims.(Cl. 84-1.25)

This invention is concerned with a circuit for providing a vibratoeffect in a musical instrument and more particularly with an electroniccircuit for establishing a vibrato effect without modifying the actionof the tone generator.

In a musical instrument, and particularly in an organ, it is desirableto provide a vibrato effect in which the tones are frequency modulatedabove and below the nominal frequency, at a sub-audio rate. In anelectronic musical instrument in which organ-representing tones aresynthesized, it is often difiicult to provide a vibrato effect byvarying the frequency of the tone generator. For example, where thetones are derived from a plurality of oscil lators, it is difficult tovary the frequency of each to the same relative degree. Similarly, in aninstrument which utilizes a mechanical tone generator, as rotating tonewheels, there are physical problems in varying the generator speed.

It has been proposed to provide a vibrato effect by causing a phaseshift of the composite audio signal at a sub-audio or low audio rate. Inthe Hammond model M-l organ, the signal in which a vibrato effect is tobe caused is connected with a multiple section electrical delay linehaving outputs at several points along the line. A mechanical switchselects different outputs sequentially along the line. The phase shiftin the output signal provides a vibrato effect. The mechanical switch,which in practice is a capacitor having a plurality of stator plateswith a rotor which scans them sampling the variously delayed signalsderived from the delay line, is expensive and subject to mechanicalproblems.

One feature of the invention is the provision of a vibratocircuitjutilizing a phase shift shift network having a plurality ofoutputs, with an electronic switch for deriving an output signalcomprising sequential signal portions from each of the outputs of thephase shift network. Another feature of the invention is that theswitching is effected at a rate above the audible range and the relativeperiod of operation of the switch is varied.

More particularly, the electronic switch comprises a free-runningmultivibrator having two sections, each with an input connected with anoutput of the phase shift network and having a common output. Yetanother feature is that the relative period of operation of themultivibrator is varied at a sub-audio rate, as by a variable impedancedevice connected in the timing circuit thereof and controlled by anoscillator.

Further features and advantages of the invention will readily beapparent from the following specification and from the drawings, inwhich:

FIGURE 1 is a schematic circuit diagram of a vibrato circuit embodyingthe invention;

FIGURE 2 illustrates waveforms characteristic of the operation of themultivibrator switch; and

FIGURES 3 through 5 are waveforms illustrating 'operation of thecircuit.

During the following description, specific values and type'designationswill be given for many elements of the circuit. It is to be understoodthat this specific disclosure is intended merely to illustrate anoperative embodiment of the invention. Many changes and modificationswill be apparent to those skilled in the art.

As pointed out above, the vibrato circuit of the present invention isparticularly useful in a musical instrument, as an organ, of the type inwhich it is difiicult to frequency modulate the tone generatorsdirectly. However, it will be understood that the circuit could be usedwith any type of tone generator. The nature of the tone representingsignal source is not important and will not be described in detail. Thesource may include suitable filters or synthesizing networks, dependingon the type of tone generator used.

Turning now to the drawings, a phase shift network or delay line 10 hasinput terminals 11 to which the composite signal from the signal sourceis connected. A pair of outputs are derived from the phase shift networkas at 1'2 and 13, there being a phase shift between the outputsdependent on the length of the line and the frequency of the signal. Thephase of the output derived at 12 is the same as the incoming signal. Amultivibrator 1'5 acts as an electronic switch and has inputs connectedwith the two outputs of the phase shift network 10. A composite outputappears at terminals 16.

A pair of variable impedance devices 17 are connected with themultivibrator to vary the relative switching period, but not theswitching frequency, at a sub-audio rate. Thus, the output of thecircuit is made up of varying portions of unshifted and shifted signals,providing an impression of frequency modulation.

More particularly, the audio input circuit connected with terminals 11includes a series isolating resistor 20, 1800 ohms. The delay line 10 ismade up of a plurality of sections of shunt capacity 10a and seriesinductance 10b. The number of sections and electrical constants of eachare not critical so long as certain general characteristics describedbelow are met. The phase shift network is terminated in a shunt resistor21, 18,000 ohms.

Multivibrator 15 has two sections 22 and 23, each onehalf of a 12AU7dual triode. The multivibrator is designed for free-running operationand a square wave output with cycle portions of balanced or equalduration. The cathodes of the two multivibrator sections are connectedtogether and returned to ground or reference potential 24 through acommon cathode resistor 25, 10,000 ohms. Each section has a plate loadresistor, 26 and 27, 10,000 ohms, connected with a positive voltagesource or B+ volts. The plate of each triode section is connected withthe grid of the other section through a coupling circuit includingcapacitors 28 and 29, 250 pf. (picofarad). The grid of each tube isreturned to B+ through resistors 32 and 33, one megohm each.

The series combination of resistor 34, 27,000 ohms, and capacitor 35,0.01 ,uf. (microfarad) is connected between phase shift output 12 andthe plate of tube 22, and thus through capacitor 28 to the control gridof tube 23. Similarly, output 13 of the phase shifter is connectedthrough resistor 36, 18,000 ohms and capacitor 37, 0.01 ,uf., with theplate of tube 23 and through capacitor 29 with the control grid of tube22. The difference in value of series resistors 34 and 36 compensatesfor the attenuation of the signal through phase shifter 10. The tonesignal amplitudes on the control grids of the two sections of themultivibrator are the same.

The frequency or repetition rate of free-running multivibrator 15 isabove the usable audio range and preferably several times the upperaudible limit of the human ear. In the particular example given above,the repetition rate is of the order of 100,000 cycles per second. Withthe circuit thus far described, the output signal will be made up ofequal portions of delayed and undelayed signal and the effect will bethat of tones without vibrato. Waveform 42, FIGURE 2, illustrates thesignal which might be observed at the plate of either section 22 or 23of the multivibrator. As each section conducts, the signal applied toits control grid appears across common cathode resistor 25. Of course,the switching rate is so high that it does not affect the output signal.

The switching period of multivibrator 15 is varied or modulated at asub-audio rate by a pair of tubes 37 and 38 connected effectively inparallel with timing resistors 32 and 33. Tubes 37 and 38 are triodes,each one-half of a 12AU7. Tube 37 functions as a sub-audio oscillator ata frequency determined by the values of inductor 39 and capacitor 40which form a resonant tuning circuit connected with the control grid.The cathode of tube 37 is returned through a bias resistor 41, 6800ohms, to a tap on inductor 39. The frequency of the oscillator may, forexample, be of the order of to 7 cycles per second for a pleasantvibrato effect.

The space current through tube 37 varies sinusoidally with the sub-audiofrequency of oscillation. As this occurs, the effective impedancepresented by tube 37 to the timing circuit of multivibrator varies. Tube38 has its anode circuit connected across resistor 33, and its controlgrid returned to ground 24. The cathode of tube 38 is returned to thetop of oscillator bias resistor 41. Accordingly, tube 38 also has asinusoidally varying space current and impedance, 180 out of phase withtube 37. As the shunt impedance in one timing circuit is increased, thatof the other circuit is decreased. Thus, the repetition rate of themultivibrator remains unchanged while the relative switching period, orthe length of time signals are derived from either output, variessinusoidally at the sub-audio frequency of control oscillator 17.

The amplitude of the current flow through tubes 37 and 38 determines therelative effect of the impedance change on the multivibrator circuit.Preferably, a maximum ratio of 9 to 1 is utilized. The switchingwaveform at 42 has a 1 to 1 ratio. Waveforms 43 and. 44 representconditions of maximum and minimum impedance of tubes 37 and 38, withswitching periods wherein one tube of the multivibrator is conductivefor W of the multivibrator period and the other tube is conductive forof the period. This switching period variation, together with a phaseshifter having a phase shift of the order of 360 at 1100 cycles persecond provides a very pleasant vibrato effect.

FIGURES 3 through 5 are waveforms illustrating the operation of aspecific embodiment of the invention utilizing a delay line having 360phase shift at 1200 cycles per second, with an audio frequency of 400cycles persecond. A switching frequency of the order of two times theaudio frequency is shown for the sake of clarity. In practice theswitching frequency might be much higher, as of the order of 100kilocycles.

In each figure, curve 50, which is positive going at the left of thefigure, represents the unshifted signal appearing at output 12 whilecurve 51 represents the shifted signal appearing at output 13. Thesecurves are shown with light lines. The portions of each signal coupledto the common output circuit by the switch are indicated by heavierlines. In FIGURE 3 switching is carried out at one extreme, with a 9:1ratio of direct to delayed signal. In FIGURE 4, the ratio is reversed,1:9 for direct to delayed signal. In FIGURE 5, the ratio is 1:1. Curves52, 52a and 52b illustrate the switching action in FIGURES 3 through 5respectively and show the signal portions selected from the two outputs,

The human ear cannot respond to rapid changes in the signal and theapparent effect on the listener is a composite or average of the signalportions derived from the outputs of the delay line. This is illustratedby the heavy line curves 53, 53a and 53b. A comparison of FIG- URES 3, 4and 5 shows that the phase of curves 53, 53a and 53b vary with differentswitching ratios. As pointed out in the specification, the switchingratio is varied at a sub-audio rate. This results in the phase shiftshown and an apparent frequency shift or vibrato effect.

I claim:

1. A vibrato circuit for a musical instrument, comprising: a phase shiftnetwork having a signal input terminal and at least two outputs; aswitch connected with the outputs of said phase shift network forderiving a composite output signal comprising sequential signal portionsfrom each of said outputs; and means for varying the relative period ofoperation of said switch to vary the signal portion derived from eachoutput.

2. The vibrato circuit of claim 1 wherein the relative period of switchoperation has a maximum ratio of the order of 9 to 1.

3. The vibrato circuit of claim 1 wherein said phase shift networkprovides a shift of the order of 360 at a frequency of 1100 cycles persecond.

4. A vibrato circuit for a musical instrument, comprising: a phase shiftnetwork having a signal input terminal and at least two outputs; afree-running multivibrator switch having two sections, each with aninput, said multivibrator having a single output; means connecting oneof the inputs of the multivibrator switch with one output of said phaseshift network; means connecting the other input of the multivibratorswitch with the other output of the phase shift network; and a controldevice connected with said multivibrator for varying the relative periodof operation thereof to vary the duration of the signal portion derivedfrom each output.

5. The vibrato circuit of claim 4 wherein the multivibrator has twosections with two intersection coupling circuits and the control deviceincludes a pair of variable impedance elements, one connected with eachof the intersection coupling circuits of the multivibrator, and meansfor periodically varying the impedance of said elements.

6. The vibrato circuit of claim 5 wherein one of said control devicesand a first impedance element form an oscillator having a bias elementin the circuit thereof, and said second variable impedance element is anamplifier having a control electrode connected with said bias element,the impedance of the second element varying oppositely and insynchronism with the variations of the impedance of the first.

7. A vibrato circuit for a musical instrument, comprising: a phase shiftnetwork having a signal input terminal and at least two outputs; afree-running multivibrator switch having two sections, each with acontrol electrode and an output electrode; means connecting the controlelectrode of one section of the multivibrator switch with one output ofsaid phase shift network; means connecting the control electrode of theother section of the multivibrator switch with the other output of thephase shift network; a common output circuit connected with the outputelectrodes of both sections of the multirvibrator; and a control deviceconnected with said multivibrator for varying the relative period ofoperation thereof to vary the duration of the signal portion derivedfrom each output.

8. A vibrato circuit for a musical instrument, comprising: a phase shiftnetwork having a signal input terminal and at least two outputs; afree-running multivibrator switch having two sections each with acontrol electrode and an output electrode and two intersection couplingcircuits connected between the two sections of the multivibrator; meansconnecting the control electrode of one section of the multivibratorswitch with one output of said phase shift network; means connecting thecontrol electrode of the other section of the multivibrator switch withthe other output of the phase shift network; a common output circuitconnected with the output electrodes of both sections of themultivibrator; a pair of variable impedance elements, one connected witheach of the intersection coupling circuits of the multivibrator; andcontrol means, forming with a first of said impedance elements anoscillator having a bias element in the circuit thereof, said secondvariable impedance element comprising an amplifier having a controlelectrode connected with said bias element, the impedance of the secondvariable impedance element changing oppositely and in synchronism withthe impedance of the first, to vary the relative period of operation ofthe switch and the duration of the signal portion derived from eachoutput.

9. A vibrato circuit for a musical instrument, comprising: a phase shiftnetwork having a signal input terminal and at least two outputs; anelectronic switch connected with the outputs of said phase shift networkfor deriving a composite output signal comprising seguential signalportions from each of said outputs; means establishing a switching rateof at least several times the audio frequency of said instrument; andmeans for varying the relative period of operation of said switch at asub-audio rate to vary the signal portion derived from each output.

References Cited by the Examiner UNITED STATES PATENTS Hanert 841.25

Stefanov 33214 X Wood 33214 X Martin et al 307'88.5 X K'abell.

Cooper et al. 33214 X Weinstein.

1O ARTHUR GAUSS, Primary Examiner.

I. C. EDELL, Assistant Examiner.

1. A VIBRATOR CIRCUIT FOR A MUSICAL INSTRUMENT, COMPRISING: A PHASESHIFT NETWORK HAVING A SIGNAL INPUT TERMINAL AND AT LEAST TWO OUTPUTS; ASWITCH CONNECTED WITH THE OUTPUTS OF SAID PHASE SHIFT NETWORK FORDERIVING A COMPOSITE OUTPUT SIGNAL COMPRISING SEQUENTIAL SIGNAL PORTIONSFROM EACH OF SAID OUTPUTS; AND MEANS FRO VARYING THE RELATIVE PERIOD OFOPERATION OF SAID SWITCH TO VARY THE SIGNAL PORTION DERIVED FROM EACHOUTPUT.